1. Field of the Invention
This invention relates to liquid crystal displays and, more specifically, to liquid crystal displays having a relatively wide viewing angle and wherein light travels through the display from rear to front as opposed to entering from the front and being reflected in the display back to the front thereof.
2. Brief Description of the Prior Art
Twisted nematic crystal type liquid crystal displays usually have very narrow viewing angles. In these displays, polarized light is blocked by crystals which are twisted under the influence of an electric field. All light exiting the envelope will be polarized at the same angle. When the viewer moves off axis, due to the acute angle at which the light is travelling, such light reaching his eye will not have passed through the entire length of the twisted crystal so as to provide full attenuation. This is due to the thickness of the crystal material which is many molecules thick. The twist is imposed upon a stack of molecules, the twist angle varying from zero at the top and bottom of the stack to a full ninety degrees at the center of the stack. Also, some new designs use crystals that are twisted through more than one turn to an odd multiple of ninety degrees, such as, for example, 270 degrees. Accordingly, there may be more than one point at which the molecules are ninety degrees displaced from the molecules in the polarizing filter. By applying an electric field which causes crystals in a defined region to twist ninety degrees, polarized light can be selectively passed or blocked in selected different regions. The regions can be arranged in patterns, causing the appearance of an image. Accordingly, when the viewer moves off the display axis, light passing through the crystals to the viewer's eye no longer passes through the entire crystal or the region of ninety degree twist and is not fully attenuated, thereby causing the image to disappear.
Most improvements in viewing angle have been achieved through refinements in the dimensions of the display, by experimenting with various crystal structures, and by increasing the duty cycle of multiplexed driving circuitry.
Providing a crystal which is as thin as possible would appear to limit the amount of off axis light available, however, parallelism of the panes of glass comprising the envelope encompassing the liquid crystal becomes a process problem and limits the size of display that can be produced with acceptable yields.
Crystal structures with faster twisting response times and slower decay times usually improve contrast ratio for a given duty cycle. However, such structures have not provided much improvement in the above noted viewing angle problem.
Increasing of the duty cycle of the drive circuits can improve the contrast ratio on the axis. Therefore, as the viewer moves off axis, even though light attenuation decreases in the same proportion, the image remains along with viewable contrast at a greater angle. However, increase of the duty cycle also requires additional circuitry and additional power.
It is therefore readily apparent that prior art attempts to solve the problem of improving the viewing angle of liquid crystal displays have not met with adequate success.